Stabilized polypropylene compositions containing a phenolic antioxidant and tert-alkyl ester of thiodisuccinic acid



United States Patent 10 Claims ABSTRACT OF THE DISCLOSURE An antioxidantcomposition for polypropylene containing (1) from about 0.001% to 5.0%by weight of said polypropylene the phenolic compound (a) 4,4-dithiobis(2,6-methylbenzylphenol), (b) 4,4'-dithiobis(ortho-phenylphenol), (c)4,4'-butylidene bis(6-tert-butyl-m-cresol), (d)4,4-thiobis(3-methyl-6-tert-butylphenol), (e) the 3:1 condensationproduct of 3-methyl-6-tert-butylphenol with crotonaldehyde or (f)6-(4-hydroxy-3,S-di-tert-butylanilino)-2,4-bis(n-0ctylthio-1,3,5-triazineand (2) from about 0.001% to about by weight of said polypropylene of atetra-alkyl ester of thiodisuccinic acid of the formula wherein R isselected from the group consisting of alkyl and phenyalkyl groupscontaining from 4 to 22 carbon atoms, inclusive. An example of saidester is tetra-n-decyl thiodisuccinate.

This application is a continuation-in-part application of applicationSer. No. 634,440, filed Apr. 26, 1967, now abandoned, which is acontinuation-in-part of application Ser. No. 347,358, filed Feb. 26,1964, now abandoned.

This invention relates to the improved stabilization of polymericmaterials containing phenolic antioxidants against oxidation by theinclusion therein of small but eifective amounts of certain esters ofthiodisuccinic acid. This invention also relates to compositions ofmatter comprising plastic resins and a phenolic or hindered phenolicantioxidant which contains one or more of certain esters ofthiodisuccinic acid functioning therein as an antioxidant.

The mechanism of oxidative deterioration toward which the stabilizercompositions of this invention are directed is known as thermaloxidation. This type of oxidative deterioration may take place in theabsence of ultraviolet light and, as the name implies, is accelerated byan increased in temperature.

Polymeric compositions which are benefited by the inclusion ofadditives, which function as antioxidants, include all of theessentially saturated hydrocarbon polymeric materials, polymerscontaining unsaturated linkages, vinylic resins, such as polyvinylchloride, and polyethers, especially of the type polyoxymethylene. Theessentially saturated hydrocarbon polymers include, especially, polymerscontaining tertiary hydrogen atoms such as polyethylene wherein randomtertiary hydrogen atoms result from branching, and particularlypolypropylene containing ordered tertiary hydrogen atoms. Polymers thatcan also be included are almost completely linear polymers 3,433,762Patented Mar. 18, 1969 such as low pressure polyethylene and materialscontaining only secondary hydrogen atoms such as polyisobutylene.Polymers containingolefinic double bonds also are extremely susceptibleto oxidation. Examples of such unsaturated polymers includepolybutadiene, polyisoprene, and the high impact plastics such as highimpact styrene and high impact polyvinyl chloride. The mechanisminvolved in the oxidation of unsaturated polymers essentially requiresperoxides and peroxide radical formation which subsequently lead tochain splitting with the formation of aldehydes and acids. Polyvinylchloride and chlorine-containing polymers in general degrade by adehydrohalogenation which produces unsaturation in the polymer. Thisprocess is usually accompanied by oxidation at the point ofunsaturation, therefore the inclusion of an antioxidant in theformulation of such compositions is extremely useful. Polyethers such aspolyoxymethylene and the like are suspectible to oxidation at elevatedtemperatures. Other materials protected by antioxidants includecopolymers as well as homopolymers and mixtures of polymers, copolymersor both. Among the copolymers of value are the ordered, block, graft andrandom copolymers.

The effect of oxidative deterioration on polymeric materials can lead tochanges in physical properties such as color, weight, hardness, tensilestrength, continuity, shape internal stresses, and odor. The process ofdegradation is undoubtedly begun by the degradation of the polymeritself, although the total length of this procedure is unknown. Ingeneral, oxidative breakdown is a two-step process: 1) a primaryoxidation of a susceptible link in the polymer chain, such as at atertiary hydrogen atom or unsaturated linkage, if one is present in thepolymer chain, and (2) a secondary procedure in which the links sooxidized attack another portion of the chain so as to result in a rapidautocatalytic breakdown of the polymer. Therefore, it is believed thefunction of any added material is to combine somehow with the oxidationproduct of the primary step so as to deactivate it and prevent furtherattack to the polymer chain, thus stopping the autocatalytic breakdown.The antioxidant is not expected to interfere With the primary oxidationstep, although this action is not particularly undesirable.

As might be expected, the seriousness of this problem has resulted inthe development and exploitation of a considerable assortment ofstabliized polymeric compositions. Among this assortment of thermalantioxidants are the secondary amines of resonant structures, phenolsand hindered phenols, and thiodipropionates. As antioxidants thephenolic class holds the greater interest. The term antioxidantstabliizer will be used hereinafter to mean stabilizers of the followingphenolic class; alkylated phenols, alkylated diphenols, thiobis(alkylated phenols). These compounds are well known to those skilledin the art to which the present invention pertains. To improve theproperties of phenolic antioxidant stabilizers various derivatives havebeen prepared and used either alone or in combination withthiodipropionic acid esters. Stabilizers containing metals and metalsalts have been suggested. The use of activated carbon black is alsomentioned in prior art. The requirement for improved stabilizers andpolymer compositions is ever continuing with the growing application ofplastics.

In accordance with the present invention, it has been discovered thatthe combination of phenolic antioxidants and the esters ofthiodisuccinic acid constitute a preferred antioxidant stabilizer forthe control of thermal oxidative degradation in oxidative susceptiblepolymer compositions as hereinabove described, and that in saidcombination the compounds are mutually activating to ac complish animproved result.

More specifically, this invention relates to the use of combinations ofphenolic and hindered phenolic polymer antioxidants and of esters ofthiodisuccinic acid of the general formula H II R-O O C-dJ-S-(B-G ORR-OOC-GH H COOR wherein R is selected from the group consisting of alkyland phenalkyl groups containing from 4 to approxiamtely 22 carbon atoms,inclusive, for the improved stabilization of oxidative deterioration topolymeric materials in which they are incorporated.

The esters of thiodisuccinic acid that were tested in combination withvarious commercial phenolic antioxidants included: tetra-n-butylthiodisuccinate, tetra-n-octyl thiodisuccinate, tetra-iso-decylthiodisuccinate, tetra-ndodecyl thiodisuccinate, tetra-iso-octylthiodisuccinate, tetra-n-decyl thiodisuccinate and tetra-benzylthiodisuccinate. The above mentioned compounds were used in combinationwith various phenolic antioxidants. Examples of said phenolicantioxidants include Topanol CA [a 3:1 condensation product of3-methyl-6-tert-butylphenol with crotonaldehyde], Irganox 565[6-(4-hydroxy 3,5 di-tert butylanilino) 2,4-bis (n-octylthio)-1,3,5-triazine], Santowhite Powdei [4,4-butylidene-bis(6-tert-butyl-meta-cresol)], Santonox R [4,4-thio-bis(3-methyl-6-tert-butylphenol) 4,4-dithio-bis(orthophenylphenol) and4,4'-dithio-bis(2,6-ot-methylbenzylphenol).

These phenolic compounds are termed antioxidants because in ordinary usetheir inhibitive active is directed to the control of oxidativedeterioration in polymers.

The compounds of the present invention may be employed by mixing withthe polymer in any suitable manner that will effect thoroughdistribution. This can be accomplished in a machine suitable for mixingsolids or incorporating liquids. Instead of adding the combination tothe polymer in the solid or molten state, it can be added to a solutionor suspension of polymer in an organic solvent or to an aqueousdispersion thereof and the volatile solvent subsequently removed byvaporization. The amount of agents added can vary from 0.001% to 5.0%based on the weight of polymer, but it is preferred to use the minimumamount required to achieve the desired results.

The following example illustrates method of testing and the utility ofthis invention.

Test methods.-The test methods employed estimate the oxidative stabilityof an essentially saturated hydro carbon polymeric material by exposureto long term oven aging at elevated temperatures below the melting pointof the resin. Polypropylene was chosen as the test polymeric materialsince it contains ordered tertiary hydrogen atoms which makepolypropylene extremely susceptible to thermal oxidation. Unstabiliz/edpolypropylene resins failed within approximately one hour in the test,while stabilized resins exhibited longer lives.

Circular specimens 1" in diameter are cut from a compression moldedsheet, prepared by pressing between polished press plates. Samplethickness was approximately 25 mils. The samples were placed in openPyrex glass petri dishes and then placed in a heated oven. The heatedoven was provided with a means of circulating air internally at auniform velocity. The temperature of the air was held at 150 C. with atolerance of :1 C.

Duplicate samples were placed in the forced air circulating ovenmaintained at the specified temperature and the initial time wasrecorded. The samples were examined every hour initially and thosewithstanding the test for more than one day were then examined threetimes a day for signs of failure. Failure was determined visually andwas defined as the start of resin degradation as shown by brown oryellow discoloration. There was also a loss of physical properties asshown by cracking and embrittlement. The failure time was determined andrecorded. A set of control or reference resins stabilized andunstabilized were run in each month. In the following comparativeexamples the number of hours to embrittlement, i.e., failure ofantioxidant, is reported.

EXAMPLE This example demonstrates the improved antioxidative stabilityeffect of combinations of selected phenolic antioxidants and certainesters of thiodisuccinic acid. The above mentioned test procedure wasemployed in this example. The table below lists the effects obtained bythe inclusion of various amounts of antioxidant on the thermal oxidativestability of polypropylene. The various combinations are to be comparedwith the effect obtained by the individual known antioxidants.

The following is a list of commercially available phenolic antioxidantsemployed in this example. Numbers have been assigned to the materialsand are used in the table to designate the phenolic antioxidant.

( 1) 4,4-dithio-bis (2,6-a-methylbenzylphenol) (2) 4,4'-dithio-bis(ortho-phenylphenol) (3) Topanol CA: a 3:1 condensation product of 3-methyl-6-tert-butylphenol with crotonaldehyde.

(4) Irganox 565:6-(4-hydroxy-3,S-di-tert-butylanilino)-2,4-bis(n-octylthio) 2,4bis(n-octylthio)-1,3,5-triazlne.

(5) Santowhite Powder: 4,4'-butylidene-bis(6-tertbutyl-m-cresol) (6)Santonox Rz 4,4'-thio-bis(3-methyl-6-tert-butylphenol).

the following esters of thiodisuccinic acid have been prepared andtested:

Designation Tetra-isodecyl thiodisuccinate Tetra-n-dodecylthiodisuccinate Tetra-n-octyl thiodisuccinate Tetra-isooctylthiodisuccinate Tetra-n-decyl thiodisuccinate Tetra-n-octadecylthiodisuccinate Tetra-benzyl thiodisuccinate B C D E F G TABLEAntioxidants at 150 0. (hrs.)

322999992999 H qqcowwr-n-nwwwuwr-nw 99999;? 99. 9. uwweemcow mtnmcocncnwThe above examples show that the compositions of the present invention,and especially the tetra-n-octyl thiodisuccinate and related esters,have been found to be outstanding in their ability to impart in the testpolymer in which they are included, protection from degradation onexposure to thermal oxidative conditions.

Various changes and modifications may be made without departing from thespirit and scope of the invention described herein as will be apparentto those skilled in the art to which it pertains. It is accordinglyintended that the present invention shall only be limited by the scopeof the appended claims.

We claim:

1. A composition of matter comprising (1) polypropylene (2) from about0.001% to 5.0% by weight of said polypropylene of a phenolic antioxidantselected from the group consisting of (a)4,4-dithio-bis(2,6-a-methylbenzylphenyl), (b)4,4'-dithio-bis(ortho-phenylphenol), (c) 4,4 butylidene bis(6 tert butylm cresol), (d) 4,4 thio bis(3 methyl 6 tert butylphenol), (e) the 3:1condensation product of 3-methyl-6-tert-butylphenol with crotonaldehyde,and (f) 6-(4-hydroxy-3,5-di-tertbuty1-anilino)-2,4-bis (n octylthio)1,3,5-triazine and (3) from about 0.001% to about by weight of saidpolypropylene of a tetra-alkyl ester of thiodisuccinic acid of theformula H H R-OOC-C-S-C-COOR R-OOC- H H COOR wherein R is selected fromthe group consisting of alkyl and phenalkyl groups containing from 4 to22 carbon atoms, inclusive.

2. The composition of matter of claim 1 wherein said tetra-alkyl esterof thiodisuccinic acid is the compound tetra-n-octyl thiodisuccinate.

3. The composition of matter of claim 1 wherein said tetra-alkyl esterof thiodisuccinic acid is the compound tetra-n-decyl thiodisuccinate.

4. The composition of matter of claim 1 wherein said tetra-alkyl esterof thiodisuccinic acid is the compound tetra-n-dodecyl thiodisuccinate.

5. The composition of matter of claim 1 wherein said tetra-alkyl esterof thiodisuccinic acid is the compound tetra-benzyl thiodisuccinate.

6. The composition of matter of claim 1 wherein the phenolic antioxidantis the compound 4,4-dithio-bis(2,6 methyl-benzylphenol) 7. Thecomposition of matter of claim 1 wherein the phenolic antioxidant is thecompound 4,4'-dithio-bis(orthophenylphenol) 8. The composition of matterof claim 1 wherein the phenolic antioxidant is the compound4,4'-butylidene-bis 6-tert-butyl-m-cresol 9. The composition of matterof claim 1 wherein the phenolic antioxidant is the 3:1 condensationproduct of 3-methyl-6-tertbutylpheno1 with crotonaldehyde.

10. The composition of matter of claim 1 wherein the phenolicantioxidant is the compound 6-(4-hydroxy-3,5-di-tert-butyl-anilino)-2,4-bis(n octylthio) 1,3,5 triazine.

References Cited UNITED STATES PATENTS 2,603,616 7/1952 Newton.2,831,897 8/1958 Harris et a1. 3,196,185 7/1965 Ranson. 3,255,136 6/1966Hecker et al. 3,245,992 4/1966 Dexter et a1.

JAMES A. SEIDL-ECK, Primary Examiner. U. P. HOKE, Assistant Examiner.

U.S. C1. X.R. 260-4585, 45.95

